Rechargeable batteries are cells which can be repeatedly charged to an electric capacity and then selectively discharged. A common element of rechargeable batteries is nickel, such as in nickel-cadmium (Ni--Cd) batteries. The charging phase of a nickel battery to a specific capacity is typically controlled by a microprocessor and charging is terminated upon the occurrence of one of four events: (1) "Voltage Cut-off": when a terminal cell voltage is reached, the charging is stopped; (2) "Temperature Cut-off": when the absolute temperature of a cell is outside a "proper charging window" (generally 5-50.degree. C.), the charging is stopped; (3) "Coulomb Counting": if the energy put into the battery by the charger exceeds a predetermined amount, the charging is stopped; and (4) "Temperature inflection."
The detection of temperature inflection method for effecting charging termination utilizes the fact that, as a battery charges, its temperature increases. From the initial charge to nearly a full charge, the temperature of the nickel battery rises at a moderate rate. When the battery reaches full capacity, the temperature begins to rise faster. This increase in temperature rise is called an "inflection," and thus, when an inflection is detected, charging should be terminated as the battery is at full capacity.
Temperature inflection is the most efficient indicator of a fully charged battery. And typically, temperature cut-off, voltage cut-off, and Coulomb counting are used as secondary indicators that the battery is fully charged. Unfortunately, the use of temperature inflection to detect the charge level of the battery is not accurate in all environments. Consequently, in mobile battery chargers, such as those used in vehicles, extreme and changing ambient temperatures can interfere with this normal charging method of using temperature inflection to determine when to cease charging the battery.
There are several problems that arise particularly in vehicular chargers due to the elevated and fluctuating temperatures which can be present. One problem is a rising battery temperature due to rising ambient temperature, which causes a false temperature inflection. Another problem with high ambient temperatures is that if the ambient temperature is close to, but does not exceed the maximum charging temperature, a temperature cut-off termination will occur. The early charging termination causes an insufficient charge to be placed within the battery.
Accordingly, it would be advantageous to have a method of charging a rechargeable battery, and particularly a nickel battery, which allows adequate charging of the battery under the various environmental conditions that mobile chargers encounter. It is therefore to the provision of such an improved method of charging a rechargeable battery in a mobile charger that the present invention is primarily directed.